Effects of the delta-opioid agonist SNC80 on the abuse liability of methadone in rhesus monkeys: a behavioral economic analysis

Xylazine does not enhance fentanyl reinforcement in rats: A behavioral economic analysis

The adulteration of illicit fentanyl with the alpha-2 agonist xylazine has been designated an emerging public health threat. The clinical rationale for combining fentanyl with xylazine is currently unclear, and the inability to study fentanyl/xylazine interactions in humans warrants the need for preclinical research. We studied fentanyl and xylazine pharmacodynamic and pharmacokinetic interactions in male and female rats using drug self-administration behavioral economic methods. Fentanyl, but not xylazine, functioned as a reinforcer under both fixed-ratio and progressive-ratio drug self-administration procedures. Xylazine combined with fentanyl at three fixed dose-proportion mixtures did not significantly alter fentanyl reinforcement as measured using behavioral economic analyses. Xylazine produced a proportion-dependent decrease in the behavioral economic Q0 endpoint compared to fentanyl alone. However, xylazine did not significantly alter fentanyl self-administration at FR1. Fentanyl and xylazine co-administration did not result in changes to pharmacokinetic endpoints. The present results demonstrate that xylazine does not enhance the addictive effects of fentanyl or alter fentanyl plasma concentrations. The premise for why illicitly manufacture fentanyl has been adulterated with xylazine remains to be determined.

Latent factor structure of a behavioral economic marijuana demand curve

RATIONALE

Drug demand, or relative value, can be assessed via analysis of behavioral economic purchase task performance. Five demand indices are typically obtained from drug purchase tasks.

OBJECTIVES

The goal of this research was to determine whether metrics of marijuana reinforcement from a marijuana purchase task (MPT) exhibit a latent factor structure that efficiently characterizes marijuana demand.

METHODS

Participants were regular marijuana users (n = 99; 37.4% female, 71.5% marijuana use days [5 days/week], 15.2% cannabis dependent) who completed study assessments, including the MPT, during a baseline session. Principal component analysis was used to examine the latent structure underlying MPT indices. Concurrent validity was assessed via examination of relationships between latent factors and marijuana use, past quit attempts, and marijuana expectancies.

RESULTS

A two-factor solution was confirmed as the best fitting structure, accounting for 88.5% of the overall variance. Factor 1 (65.8% variance) reflected "Persistence," indicating sensitivity to escalating marijuana price, which comprised four MPT indices (elasticity, O max, P max, and breakpoint). Factor 2 (22.7% variance) reflected "Amplitude," indicating the amount consumed at unrestricted price (intensity). Persistence factor scores were associated with fewer past marijuana quit attempts and lower expectancies of negative use outcomes. Amplitude factor scores were associated with more frequent use, dependence symptoms, craving severity, and positive marijuana outcome expectancies.

CONCLUSIONS

Consistent with research on alcohol and cigarette purchase tasks, the MPT can be characterized with a latent two-factor structure. Thus, demand for marijuana appears to encompass distinct dimensions of price sensitivity and volumetric consumption, with differential relations to other aspects of marijuana motivation.

Molecular Pharmacology of δ-Opioid Receptors

Opioids are among the most effective analgesics available and are the first choice in the treatment of acute severe pain. However, partial efficacy, a tendency to produce tolerance, and a host of ill-tolerated side effects make clinically available opioids less effective in the management of chronic pain syndromes. Given that most therapeutic opioids produce their actions via µ-opioid receptors (MOPrs), other targets are constantly being explored, among which δ-opioid receptors (DOPrs) are being increasingly considered as promising alternatives. This review addresses DOPrs from the perspective of cellular and molecular determinants of their pharmacological diversity. Thus, DOPr ligands are examined in terms of structural and functional variety, DOPrs' capacity to engage a multiplicity of canonical and noncanonical G protein-dependent responses is surveyed, and evidence supporting ligand-specific signaling and regulation is analyzed. Pharmacological DOPr subtypes are examined in light of the ability of DOPr to organize into multimeric arrays and to adopt multiple active conformations as well as differences in ligand kinetics. Current knowledge on DOPr targeting to the membrane is examined as a means of understanding how these receptors are especially active in chronic pain management. Insight into cellular and molecular mechanisms of pharmacological diversity should guide the rational design of more effective, longer-lasting, and better-tolerated opioid analgesics for chronic pain management.

Corn oil, but not cocaine, is a more effective reinforcer in obese than in lean Zucker rats

Obesity is associated with abnormal brain reactivity in response to palatable food consumption, a factor that may contribute to non-homeostatic eating. However, little is known about how obesity interacts with the reinforcing effects of highly palatable constituents of food (e.g., fat), and if altered reinforcement processes associated with obesity generalize to non-food reinforcers. The current study compared the reinforcing effects of a fat (corn oil) and a drug of abuse (cocaine) in obese and lean Zucker rats. Specifically, obese and lean Zucker rats self-administered corn oil or intravenous cocaine in a behavioral economic demand procedure. For corn oil, maximum demand was higher and demand elasticity was lower in the obese rats compared to their lean counterparts. However, there were no differences in demand for cocaine between the obese and lean rats. These results demonstrate that a fat in the form of corn oil is a more effective reinforcer in obese Zucker rats. However, the fact that demand for cocaine was not different between the obese and lean rats suggests that differences in reward mechanisms may be reinforcer-specific and do not necessarily generalize to non-food reinforcers.

Kir3 channel signaling complexes: focus on opioid receptor signaling

Opioids are among the most effective drugs to treat severe pain. They produce their analgesic actions by specifically activating opioid receptors located along the pain perception pathway where they inhibit the flow of nociceptive information. This inhibition is partly accomplished by activation of hyperpolarizing G protein-coupled inwardly-rectifying potassium (GIRK or Kir3) channels. Kir3 channels control cellular excitability in the central nervous system and in the heart and, because of their ubiquitous distribution, they mediate the effects of a large range of hormones and neurotransmitters which, upon activation of corresponding G protein-coupled receptors (GPCRs) lead to channel opening. Here we analyze GPCR signaling via these effectors in reference to precoupling and collision models. Existing knowledge on signaling bias is discussed in relation to these models as a means of developing strategies to produce novel opioid analgesics with an improved side effects profile.

Assessment of the effects of contingent histamine injections on the reinforcing effectiveness of cocaine using behavioral economic and progressive-ratio designs

RATIONALE

Recent research has demonstrated that the drug, histamine, can function as a punisher of cocaine self-administration. However, little is known about how drug punishers affect the maximum reinforcing effectiveness of drugs as reinforcers.

OBJECTIVE

The goal of the present study was to determine if histamine, when self-administered as a mixture with cocaine, could reduce cocaine's maximum reinforcing effectiveness using two procedures designed for measuring reinforcing effectiveness.

METHODS

In the first experiment, rhesus monkeys were allowed to self-administer cocaine (0.1 mg/kg/inj) alone or as a mixture with histamine (0.012-0.05 mg/kg/inj) in a behavioral economic design. In the second experiment, monkeys were allowed to self-administer cocaine alone (0.006-0.56 mg/kg/inj) or as a mixture with histamine (0.025-0.1 mg/kg/inj) under a progressive-ratio schedule of reinforcement.

RESULTS

In Experiment 1, histamine decreased the reinforcing effectiveness of cocaine in a dose-dependent manner as evidenced by increases in cocaine's demand elasticity with increases in histamine dose. In Experiment 2, histamine decreased cocaine's potency and effectiveness as a reinforcer in a dose-dependent manner as indicated by rightward and downward shifts, respectively, in the dose-response functions.

CONCLUSION

The reinforcing effectiveness of cocaine can be reduced by contingent self-administration of histamine. These results indicate that combining drug punishers with drug reinforcers reduces the maximum reinforcing effect of the drug reinforcer, which suggests a use for drug punishers as a deterrent to drug abuse (e.g., as mixtures with prescription medications with abuse potential).

Behavioral economics and empirical public policy

The application of economics principles to the analysis of behavior has yielded novel insights on value and choice across contexts ranging from laboratory animal research to clinical populations to national trends of global impact. Recent innovations in demand curve methods provide a credible means of quantitatively comparing qualitatively different reinforcers as well as quantifying the choice relations between concurrently available reinforcers. The potential of the behavioral economic approach to inform public policy is illustrated with examples from basic research, pre-clinical behavioral pharmacology, and clinical drug abuse research as well as emerging applications to public transportation and social behavior. Behavioral Economics can serve as a broadly applicable conceptual, methodological, and analytical framework for the development and evaluation of empirical public policy.

Effects of 14-day treatment with the schedule III anorectic phendimetrazine on choice between cocaine and food in rhesus monkeys

BACKGROUND

The clinical utility of monoamine releasers such as phenmetrazine or d-amphetamine as candidate agonist medications for cocaine dependence is hindered by their high abuse liability. Phendimetrazine is a clinically available schedule III anorectic that functions as a prodrug for phenmetrazine and thus may have lower abuse liability. This study determined the effects of continuous 14-day treatment with phendimetrazine on cocaine vs. food choice in rhesus monkeys (N=4).

METHODS

Responding was maintained under a concurrent schedule of food delivery (1-g pellets, fixed-ratio 100 schedule) and cocaine injections (0-0.1mg/kg/injection, fixed-ratio 10 schedule). Cocaine choice dose-effect curves were determined daily before and during 14-day periods of continuous intravenous treatment with saline or (+)-phendimetrazine (0.32-1.0mg/kg/h). Effects of 14-day treatment with (+)-phenmetrazine (0.1-0.32 mg/kg/h; N=5) and d-amphetamine (0.032-0.1mg/kg/h; N=6) were also examined for comparison.

RESULTS

During saline treatment, food was primarily chosen during availability of low cocaine doses (0, 0.0032, and 0.01 mg/kg/injection), and cocaine was primarily chosen during availability of higher cocaine doses (0.032 and 0.1mg/kg/injection). Phendimetrazine initially decreased overall responding without significantly altering cocaine choice. Over the course of 14 days, tolerance developed to rate decreasing effects, and phendimetrazine dose-dependently decreased cocaine choice (significant at 0.032 mg/kg/injection cocaine). Phenmetrazine and d-amphetamine produced qualitatively similar effects.

CONCLUSIONS

These results demonstrate that phendimetrazine can produce significant, though modest, reductions in cocaine choice in rhesus monkeys. Phendimetrazine may be especially suitable as a candidate medication for human studies because of its schedule III clinical availability.

Behavioral economic analysis of cue-elicited craving for tobacco: a virtual reality study

INTRODUCTION

Subjective craving is a prominent construct in the study of tobacco motivation; yet, the precise measurement of tobacco craving poses several difficulties. A behavioral economic approach to understanding drug motivation imports concepts and methods from economics to improve the assessment of craving.

METHODS

Using an immersive virtual reality (VR) cue reactivity paradigm, this study tested the hypothesis that, compared with neutral cues, tobacco cues would result in significant increases in subjective craving and diverse aspects of demand for tobacco in a community sample of 47 regular smokers. In addition, the study examined these motivational indices in relation to a dual-component delay and cigarette consumption self-administration paradigm.

RESULTS

In response to the VR tobacco cues, significant increases were observed for tobacco craving and the demand indices of Omax (i.e., maximum total expenditure toward cigarettes) and Breakpoint (i.e., price at which consumption is completely suppressed), whereas a significant decrease was observed for Elasticity (i.e., lower cigarette price sensitivity). Continuous analyses revealed trend-level inverse associations between Omax and Intensity in relation to delay duration and significant positive associations between subjective craving, Omax, and Elasticity in relation to the number of cigarettes purchased.

CONCLUSIONS

The results from this study provide further evidence for the utility of behavioral economic concepts and methods in understanding smoking motivation. These data also reveal the incremental contribution of behavioral economic indices beyond subjective craving in predicting in vivo cigarette consumption. Relationships to previous studies and methodological considerations are discussed.

Endogenous opiates and behavior: 2011

This paper is the thirty-fourth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2011 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

Latent factor structure of a behavioral economic cigarette demand curve in adolescent smokers

Behavioral economic demand curves, or quantitative representations of drug consumption across a range of prices, have been used to assess motivation for a variety of drugs. Such curves generate multiple measures of drug demand that are associated with cigarette consumption and nicotine dependence. However, little is known about the relationships among these facets of demand. The aim of the study was to quantify these relationships in adolescent smokers by using exploratory factor analysis to examine the underlying structure of the facets of nicotine incentive value generated from a demand curve measure. Participants were 138 adolescent smokers who completed a hypothetical cigarette purchase task, which assessed estimated cigarette consumption at escalating levels of price/cigarette. Demand curves and five facets of demand were generated from the measure: Elasticity (i.e., 1/α or proportionate price sensitivity); Intensity (i.e., consumption at zero price); O(max) (i.e., maximum financial expenditure on cigarettes); P(max) (i.e., price at which expenditure is maximized); and Breakpoint (i.e., the price that suppresses consumption to zero). Principal components analysis was used to examine the latent structure among the variables. The results revealed a two-factor solution, which were interpreted as "Persistence," reflecting insensitivity to escalating price, and "Amplitude," reflecting the absolute levels of consumption and price. These findings suggest a two factor structure of nicotine incentive value as measured via a demand curve. If supported, these findings have implications for understanding the relationships among individual demand indices in future behavioral economic studies and may further contribute to understanding of the nature of cigarette reinforcement.

Assessment of ropinirole as a reinforcer in rhesus monkeys

BACKGROUND

Ropinirole, a D(2)/D(3)/5-HT(1A) agonist, is used for the treatment of Parkinson's disease and restless leg syndrome, and is currently being evaluated as a treatment for cocaine dependence. However, there is little information available on ropinirole's reinforcing effects.

METHODS

The current study tested ropinirole in monkeys (n=7) trained to self administer cocaine on a fixed-ratio 25 (FR 25) schedule of reinforcement to determine if it would function as a reinforcer. In addition, a behavioral economics approach was used in four monkeys to compare the reinforcing effectiveness of ropinirole to cocaine.

RESULTS

Cocaine (0.01-0.3 mg/kg/injection) functioned as a reinforcer in all monkeys under the FR 25 schedule, and ropinirole (0.01-0.1mg/kg/injection) functioned as a reinforcer in all but one. Furthermore, cocaine was a more effective reinforcer than ropinirole as indexed by demand functions.

CONCLUSION

The current data indicate that ropinirole has reinforcing effects in monkeys, although its effectiveness as a reinforcer is relatively weak.

Interaction between Mu and Delta Opioid Receptor Agonists in an Assay of Capsaicin-Induced Thermal Allodynia in Rhesus Monkeys

Delta opioid agonists enhance antinociceptive effects of mu-opioid agonists in many preclinical assays of acute nociception, but delta/mu interactions in preclinical models of inflammation-associated pain have not been examined. This study examined interactions between the delta agonist SNC80 [(+)-4-[(αR)-α-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide] and the mu agonist analgesics methadone, morphine, and nalbuphine in an assay of capsaicin-induced thermal allodynia in rhesus monkeys. Thermal allodynia was produced by topical application of capsaicin to the tail. Antiallodynic effects of methadone, morphine, and nalbuphine were evaluated alone or in combination with fixed proportions of SNC80 identical to proportions previously shown to enhance acute thermal antinociceptive effects of these mu agonists in rhesus monkeys (0.9 : 1 SNC80/methadone; 0.29 : 1 SNC80/morphine; 3.6 : 1 SNC80/nalbuphine). Methadone, morphine, and nalbuphine each produced dose-dependent antiallodynia. SNC80 produced partial antiallodynia up to the highest dose tested (5.6 mg/kg). SNC80 produced a modest, enantioselective, and naltrindole-reversible enhancement of methadone-induced antiallodynia. However, SNC80 did not enhance morphine antiallodynia and only weakly enhanced nalbuphine antiallodynia. Overall, SNC80 produced modest or no enhancement of the antiallodynic effects of the three mu agonists evaluated. These results suggest that delta agonist-induced enhancement of mu agonist antiallodynia may be weaker and less reliable than previously demonstrated enhancement of mu agonist acute thermal nociception.